Hay=There is/There are

Use the following ways:

Hay + indefinite article (un/una/unos/unas). "En mi clase hay una chica bonita"- In my class there is a pretty girl.

Hay + mucha/mucho/muchos/muchas. "En mi clase hay muchas chicas bonitas"- In my class there are many pretty girls."

Hay + noun (without the article). "En mi clase hay chicas bonitas."- In my class there are pretty girls.

Common errors:
Use Hay + definitie article (el/la/los/las).: You don't want to say "In my class there is the pretty girl" unless everyone already knows who the pretty girl is. If you haven't mentioned her before you must say "En mi clase hay una chica bonita"